Light sensitive photothermographic or heat sensitive film generally includes a base material, such as a thin polymer or paper, which is coated, typically on one side, with an emulsion of heat sensitive material, such as dry silver. Once such film has been subjected to photostimulation to form a latent image thereon, such as via a laser of a laser imager, a thermal processor is employed to develop the latent image through application of heat. Generally, such film is processed or developed at a temperature in the vicinity of 120 degrees centigrade for a required development time. In order to produce a high quality developed image, heat transfer to the photothermographic film must be controlled during the development process. If heat transfer is not uniform during development, visual artifacts, such as non-uniform density and streaking, may occur. If heat is transferred too quickly, the base of some types of film can expand too quickly, resulting in expansion wrinkles that create visual artifacts in the developed image.
Several image processing machines have been developed for thermally processing photothermographic film in efforts to achieve optimal heat transfer to the photothermographic film during development. One type of thermal processor is commonly referred to as a drum processor which employs a rotating heated drum to transfer heat to the film as it wraps around at least a portion of a circumference of the drum during processing. One type of drum processor employs a drum which is heated by an electric blanket heater coupled to an interior surface of the drum, and a series of pressure rollers positioned about a segment of the external circumference of the drum. During development, rotation of the drum draws the photothermographic film between the drum and the pressure rollers, with the pressure rollers typically holding the emulsion side of the film in contact with the drum. As the film is wrapped around at least a portion of the exterior circumference of the drum as it passes through the processor, thermal energy is transferred from the drum to the film so as to heat and maintain the film at a desired development temperature for a desired development time.
However, during operation of the processor, heat loss from the drum is not uniform and, if not compensated for, can result in visual artifacts in the developed film. For example, during idle times (when no film is being processed), heat is lost more rapidly near the ends of the drum than in the middle portion of the drum. Conversely, during processing, because the film has a width which is less than that of the drum, as heat is transferred to the film more heat is lost from the middle portion of the drum than is lost at the ends of the drum. In attempts to maintain a uniform temperature across the width of the drum at all times, some electric blanket heaters with only a single zone are configured with a varying watt-density so as to provide more thermal energy at the drum ends as compared to the drum middle (e.g. end vs. middle watt-density). Other electric blanket heaters employ multiple, individually controllable heat zones which are controlled so as to provide more heat to the end portions of the drum during idle times and to provide more heat to the middle portion during processing.
While electric blanket heaters are effective at maintaining an even temperature across a width of the drum during both processing and idle times, blanket heaters can be expensive relative to the cost of an image processor as a whole, particularly for low volume processors (i.e. processors intended for use in environments having low volume film processing requirements). In light of the above, there is a need for a cost effective photothermographic film processor that provides even film heating during processing.